Diesel engines produce an exhaust gas including inter alia carbon monoxide (CO), hydrocarbons (HC), soot and nitrogen oxides (NOx), which NOx including nitrogen oxide (NO) and nitrogen dioxide (NO2). NO2 is toxic and can cause headaches, dizziness and nausea in low doses. It also has an objectionable smell. CO, NOx, soot and HC are legislated so that the levels emitted from a diesel exhaust system to atmosphere must meet prescribed limits. Thus a diesel oxidation catalyst catalyses the oxidative removal of HC, CO and soot as does Johnson Matthey's Continuously Regenerating Trap (CRT™) (see EP-A-0341832). Many vehicles, including buses and trains, are powered by heavy-duty diesel power plants (as defined by the relevant European, US Federal or Californian legislation), all of which met the legislation existing at the time of their production.
EP-A-0560991 and EP-A-0758713 describe processes for removing NOx from the exhaust gas from diesel engines by absorbing it on a solid absorbent while the gas is lean (i.e. lambda>1) as in normal lean-bum engine operation and regenerating the absorbent by intermittently adjusting the gas to a stoichiometric or rich composition. The intermittent adjustment requires engine inlet modification and/or reductant injection, and therefore is unattractive for existing and immediately-available vehicles because of the expense and technical complexity.
The average speed of vehicles in towns and city centres is relatively low. For example, we understand that the average speed in central London, U.K. is about 4 mph. The levels of pollutants, including NO2, in city centre locations can be relatively high, and in cities such as Manhattan in New York, “canyons” are formed between tall buildings and this can prevent polluted air from moving, mixing and becoming diluted by “fresh” air from outside these canyons.
Whilst vehicles powered by diesel power plants used in e.g. city centres meet legislative requirements for NOx, it would be desirable to go beyond the relevant legislation in certain situations in order to reduce the exposure of, e.g. passengers boarding or alighting from a bus, to NO2 and generally to reduce the level of NOx in city centres for environmental reasons.